Smallsat Surveillance Constellation Using Mimo Radar

ABSTRACT

A method is presented for the formation of a constellation of small satellites (smallsats) for radar surveillance employing multiple input, multiple output (MIMO) radar operation. Such a constellation can be used for cost-effective fine angular resolution and persistent remote sensing of targets or regions on, above, and below a planet&#39;s surface. Applications include, but are not limited to, surface mapping (including change detection), mapping of meteorological conditions, and detection of fleeting events. The method pertains to satellite configurations whose costs and aggregate masses are much less than those of conventional space-based radar measurements attempting to attain comparable angular resolution, while providing persistent surveillance.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 62/918,083, filed Jan. 11, 2019, entitled “Smallsat Surveillance Constellation Using MIMO Radar,” the entire contents of which is herein incorporated by reference, for all purposes.

TECHNICAL FIELD

The instant invention relates to the field of electromagnetic sensing from space of targets lying within a spatial region of the surface of a planet, such as Earth.

BACKGROUND OF THE INVENTION

Electromagnetic measurements made from space of targets that lie within a spatial region that extends from above, on or below the surface of a planet, such as Earth, are generally of interest to the civilian sector, the military sector, and the intelligence community. In addition, the use of radio frequencies (RF), employing radar, permits day and night observations and the ability to penetrate weather conditions such as cloud cover.

Present day surveillance of the Earth conducted from satellite platforms generally involves the use of large and heavy satellites, which requires high development and launch costs, the lack of graceful mission degradation in the event of satellite malfunction or other disability, and insufficient detection sensitivity or resolution for certain missions.

It is well known that the resolution obtainable from a radar system, that is, the minimum separation of targets that can be independently observed, is inversely related to the overall size of the radar antenna aperture.

There have been attempts in the past to use distributed arrays of RF receivers on board satellites in an effort to improve the resolution. For example, U.S. Pat. No. 4,843,397 to Galati, et al, teaches a non-MIMO distributed array of RF receivers on board satellites to form a large phased-array radar antenna (typically hundreds to thousands of wavelengths in extent), thereby attaining fine angular resolution over a large spatial region, as an alternative to employing an impractically-large conventional spaceborne radar.

It is also well known that various communication techniques can increase the capacity of a communication channel to transmit information, such as in an RF link between two points in space. One such technique is known as “multiple input, multiple output,” or “MIMO,” wherein multiple antennas with both transmitters and receivers, can send and receive multiple signals along distinct (orthogonal) signal paths, and along with sophisticated signal pre-processing and post-processing, can result in enhanced performance of the RF communication link.

In the context of a radar system, wherein the radar system provides both the transmission and the reception of the RF signals that reflect from a spatial region, the transmitting and receiving antennas may be one-and-the-same or in close mutual proximity. Past and current MIMO research and implementations comprise a large variety of radar configurations (see, e.g., Bergin, J., and Guerci, J. R., “MIMO Radar: Theory and Application,” Artech House, 2018 and Davis, M. S. and and Lanterman, A. D., “Coherent MIMO Radar: The Phased Array and Orthogonal Waveforms,” IEEE Aerospace and Electronic Systems Magazine, Part II of II, August 2014.)

U.S. Pat. No. 7,994,965 to Longstaff teaches a system for generalized MIMO-based radars involving numerous element configurations and means for attaining orthogonal signal paths. It describes how MIMO can be used, apparently aboard unmanned air vehicles (UAVs), to form a large array of “transient” (i.e., virtual) elements. As another example, U.S. Pat. No. 8,427,360 to Longstaff teaches employing MIMO radar operation for performing airborne terrain mapping using unattended air vehicles (UAVs). U.S. Pat. No. 10,145,936 to Kishigami, et al, teaches MIMO radar configurations comprising a plurality of linear transmitting arrays, with different element spacings, and a plurality of receiving arrays, with different element spacings, apparently for application to automotive radar. US Patent Application 20160204840 by Liu teaches a single linear MIMO array for use in connection with synthetic aperture radar (SAR) to obtain 3-D images, similar to those obtained with conventional interferometric SAR (IFSAR).

The term “smallsat” generally refers to small orbiting spacecraft having mass within the range from 500 kilograms down to less than one kilogram. While this is not a hard delineation, the point is that smallsats, due to their low mass and low spatial size, can be deployed into orbit in large quantities using a single launch vehicle or a few launch vehicles, or by means of prior storage aboard space stations. This leads to substantial cost savings compared to conventional satellites, while also permitting the “graceful” degradation of mission operations in the event of malfunction or destruction—caused, for example, by orbital debris. Moreover, after initial deployment of a smallsat constellation, the constellation can be further augmented by launching more smallsats. For these reasons, a constellation of smallsats can provide overall improved effectiveness as contrasted with conventional large satellite systems.

SUMMARY

A method and system are presented wherein a constellation of small satellites (“smallsats”) is equipped with radar or other electromagnetic transmitters and receivers, to be used in association with MIMO techniques to provide high resolution remote sensing of spatial regions above, below, and upon a planet's surface. In this fashion, the configuration can form a denser array of “virtual” antenna elements, thereby forming a large number of effective receiving beams, resulting in fine angular resolution while providing persistent surveillance of a large surveillance sector—while limiting the amount of actual RF hardware in orbit.

The advantages of the present method include the attainment of fine angular resolution that is otherwise available only with much costlier and heavier spacecraft systems. In addition, the method permits persistent observation for detecting fleeting events over the entire surveillance sector. The overall operation of this method moreover exploits the inherent benefits of smallsats, which include lower launch costs, graceful constellation degradation, and direct in-orbit augmentation with additional smallsats—thereby further enhancing angular resolution and detection sensitivity.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments of the invention to be described herein will be more easily understood in conjunction with the following drawings:

FIG. 1 is a generalized depiction of the smallsat-MIMO method, not drawn to scale.

FIG. 2A is a schematic representation of an illustrative “baseline” constellation.

FIG. 2B is a schematic representation of an illustrative “picture frame” constellation.

FIG. 3A is a schematic representation of the virtual array corresponding to the “baseline” illustration of FIG. 2A.

FIG. 3B is a schematic representation of the virtual array corresponding to the “picture frame” illustration of FIG. 2B.

FIG. 4A is an isometric drawing of an illustrative “docked” smallsat arrangement.

FIG. 4B is an isometric drawing of an illustrative “connected” smallsat arrangement.

FIG. 4C is an isometric drawing of an illustrative “free-flying” smallsat arrangement.

FIG. 5A is an isometric drawing illustrating a downlooking face of a transmitting smallsat.

FIG. 5B is an isometric drawing illustrating a downlooking face of a receiving smallsat.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The instant invention involves a constellation of small satellites (smallsats) for radar or other electromagnetic surveillance, in association with MIMO processing techniques, to provide high-resolution measurements of a spatial region in proximity to the surface of a planet such as Earth. Such a constellation can be employed for various applications, including, but not limited to, remote sensing and mapping of the surface, real-time land, sea, subterranean, and meteorological surveillance, and documenting important environmental changes such as glacial flow, floods, and deforestation.

More specifically, by employing a limited number of smallsats carrying actual radio frequency (RF) transmitters and receivers, and including MIMO generation and post-processing techniques, the configuration can form a virtual array which behaves, in terms of beam patterns, as an array of actual antenna elements at the same locations as the virtual elements. Thus, a large number of simultaneous narrow receiving beams can be formed, resulting in fine angular measurements of radar targets or of regions in the vicinity of a planet's surface, while providing persistent surveillance of fleeting events (e.g., earthquakes, tsunamis, tornadoes) over a dwell interval whose duration is dictated by the constellation's orbital motion.

By contrast, a non-MIMO smallsat configuration can attain similar angular resolution and persistence only by populating a larger constellation with conventional radars at the locations of the aforementioned virtual antenna elements. This would require high aggregate RF hardware costs, as well as high aggregate launch mass and launch costs, typically far in excess of those of the instant invention. In the instant invention, by properly “filling” such an “unthinned” virtual array using MIMO techniques, the formation of angular “grating lobes” (i.e., spurious undesired secondary beams) are substantially avoided.

While some radar measurements—particularly those involving surveillance of unchanging or slowly varying regions such as land terrain or icecap extent—can be carried out using synthetic aperture radar (SAR) or interferometric SAR, these methods are generally not practical for targets or regions with short coherence times. For example, when observing quickly changing events such as ocean waves, tornadoes, and other weather events, the MIMO approach described herein has much to offer beyond the use of SAR.

The configurations included in this present method are those consisting of smallsats, each smallsat containing transmitting equipment, or receiving equipment, or both. Each transmitter transmits a different waveform and each receiver receives all the target echoes corresponding to these different waveforms.

The most commonly considered transmitted MIMO waveforms are those that are “orthogonal.” By orthogonal waveforms is meant those for which their modulations are such that, ideally, the multiplicity of the modulations of the received echoes at each receiver are totally uncorrelated. It should be observed that, since orthogonal transmissions are uncorrelated, each separate transmitter normally illuminates a wide angular sector. This sector is here termed the “surveillance sector.” The size of a surveillance sector is inversely related to the directive gain of the transmitting antenna and, as a general consideration, the size of a surveillance sector as observed from space can be very large compared to airborne or surface-based surveillance.

In practice, complete orthogonality, that is, complete decorrelation, among the separately transmitted waveforms is not attainable; some “spillover” of spectral sidelobes, even if extremely small, is inevitable. Therefore, the term “orthogonal,” as used in the description of the instant invention, should be understood to mean “substantially orthogonal.”

After the orthogonal waveforms are transmitted, their reflected echoes are detected by a plurality of receivers. Within each receiver is a set of “matched filters,” each matched to a separate orthogonal echo waveform, thereby selecting that waveform and substantially blocking all others. The output of each matched filter then corresponds to a single array element of a “virtual” aperture. The selected matched filter outputs are then coherently combined in a central processor with the other matched filter outputs, in order to form surveillance beams created by this virtual aperture.

In accordance with a preferred embodiment of the instant invention, as shown in FIG. 1, a smallsat constellation 101 disposed in orbit around a planet, transmits a plurality of overlapping wide beams 102 of electromagnetic energy (e.g., RF) with orthogonal waveforms toward a planet, using MEMO techniques. Reflecting from the surface of the planet, or its vicinity, is a cluster of received beams 103, which are detected by a plurality of receivers within the constellation and further processed using MEMO techniques. Additionally depicted in FIG. 1 is a base station 104, in this case located on the earth's surface, for reporting the measurements, for generating instructions to the smallsat constellation, and for recording the post-processed data.

In the formation of the received beam cluster, amplitude and phase weights can be applied to reduce or shape beam sidelobe levels and to create pattern nulls for interference suppression.

Orthogonal waveforms used to modulate the transmitted waves can take a variety of forms. One form uses coded pulses, in which case each matched filter output represents “pulse compression,” as is common in most radar systems; in this form, the pulse duration should be short enough to avoid proliferating the number of matched filters in order to cover the spread of Doppler frequencies due to the constellation's orbital motion. The coded pulse can be repeated at range unambiguous intervals (i.e., at sufficiently low pulse repetition frequency) to allow extended coherent integration, thereby permitting the enhancement of detection sensitivity. (It should also be noted that each received coded pulse can be processed digitally.)

A second form uses separate carrier frequencies, for which the matched filters are simply ordinary band-limited electronic filters, which can be constituted digitally using, for example, the fast-Fourier transform; in this form, the carrier frequencies should be separated by more than the aforementioned Doppler frequency spread due to the orbital motion.

A preferred choice of orthogonal waveforms combines the above-mentioned forms, legally-established frequency allocations permitting. The incorporation of a coded pulse into the separated carrier frequencies permits the unambiguous determination of echo range within each virtual beam; for this choice, a single pulse code can be used for all of the separated carrier frequencies. This choice, however, requires that the carrier frequency separations be increased to maintain adequate spectral separation.

Other orthogonal waveforms may be employed, as is well known by those skilled in the art, such as time-division multiplexing.

Theoretically, as a formality, it is not required that the waves be totally orthogonal in a mathematical sense, only that there be a measurable distinction in the waveforms. (See paper by Fuhrmann, D. R. and San Antonio, G., “Transmit Beamforming for MEMO Radar Systems Using Partial Signal Correlation,” IEEE Thirty Eighth Asilomar Conference on Signals, Systems and Computers, 2004.) Although the instant invention can be operated with such a partially-correlated set of waveforms, the latter is not considered advantageous for any of the contemplated measurement applications. For the sake of completeness, however, the term “substantially orthogonal,” as used in the instant invention, is taken to include partially-correlated waveforms.

Digital processing provides the beneficial feature of readily permitting different beam formations to be computed and compared concurrently, for obtaining measurements satisfying the goals indicated above. Moreover, by recording the digitally-processed data at a base station 104, such measurement data can be used for environmental research as well as for planning subsequent measurements.

Smallsat constellations employing MIMO to attain fine resolution can be configured in many ways. FIG. 2A and FIG. 2B schematically depict two configurations, termed “baseline” and “picture-frame,” respectively. (For the depicted configurations, a small number of smallsats are shown for purposes of illustrating the method; typically, a much larger number would be employed for many applications.) It should be noted that the examples depicted are not exhaustive and do not preclude other configurations for the present invention.

As mentioned earlier, existing smallsat constellations can be augmented with the deployment of additional smallsats. Such an augmentation can (a) allow substitution of defective existing smallsats, (b) increase the detection sensitivity by increasing the aggregate transmitted power and cumulative receive aperture area, and (c) further improve the angular resolution of surveillance observations due to the resulting narrower received beams.

In various embodiments of the present method, a constellation comprises numerous smallsats, configured in a “dense” and/or “fixed” formation. By a dense formation is meant one that employs spacings between adjacent smallsats that are small enough to provide a “filled” virtual aperture and to thereby avoid grating lobes. By a fixed formation is meant that, except for unintentional perturbations, one where the locations and orientations of the individual smallsats in relation to one another remain unchanged within a particular constellation over a selected measurement interval. These spacings and fixed relative locations are in contrast with sparser spacecraft configurations presented elsewhere (Steyskal, H., et al, “Pattern Synthesis for TechSat21—A Distributed Space-Based Radar System,” IEEE Antennas and Propagation Magazine, pp. 19-25, August 2003).

In such configurations, each transmitting smallsat 201 contains, among other components, an RF power amplifier. Each receiving smallsat 202 contains, among other components, a “front-end” amplifier. Each smallsat 201, 202 also contains: an appropriate deployable radar antenna, which can be either an electronically-phase-steered subarray or a mechanically-steered antenna, as well as a deployable solar energy collector to generate prime power; and a means for storing energy. Also included is at least one in-orbit central processor 203. This central processor 203 collects and combines the coherent matched filter outputs from the receiving smallsats; it also relays commands to the constellation regarding the constellation configuration and orientation as well as the desired transmitted waveforms. In addition, one or more ground stations 104 are used for additional data processing, for reporting the measurement information and, if desired, for data recordings. Such recordings can be a valuable input for research subsequently performed regarding the surveillance measurements. (Please note that the term “central processor” refers to a central functional role, and does not necessarily connote a geometric center.)

Additionally depicted in FIG. 2A and FIG. 2B are thrusters 204 attached to the smallsat constellation. These thrusters permit a constellation to be oriented to face in any direction; they also enable a constellation to maintain a planet-centered orientation as it proceeds along its orbit. The latter capability, using thrusters to control the entire constellation, obviously does not apply to the “free-flying” smallsat arrangement discussed below in connection with FIG. 4C.

FIG. 3A and FIG. 3B schematically depict the virtual array elements 301 formed by the MIMO processing of the configurations depicted in FIG. 2A and FIG. 2B, respectively. Virtual elements are formed approximately at the midpoints of all of the possible transmitting and receiving pairings, with spacings half as close as the spacings between the actual smallsats (Davis, M. S. and Lanterman, A. D., “Coherent MIMO Radar: The Phased Array and Orthogonal Waveforms,” IEEE Aerospace and Electronic Systems Magazine, Part II of II, August 2014 and Kilpatrick, T. and Longstaff, I. D., “Generalising the co-array, for SAR and MIMO radar,” IEEE International Radar Conference Record,” Washington, D.C., May 2015). Thus, for M transmitting smallsats and N receiving smallsats, the total number of virtual array elements is the product of M and N. For example, if fifty transmitting smallsats and one-hundred receiving smallsats are employed, the total number of virtual array elements is 5000, whereas the actual number of smallsats is only one-hundred and fifty. Thus, fine resolution, and the associated high precision, is obtainable without incurring high costs of RF hardware.

However, the substantial cost savings for RF components should be balanced against the large amount of processing required to realize the advantages of the MIMO operation. This processing load may constitute the main drawback of the present method, but future advances in digital technology are expected to help to mitigate this drawback. There are, in addition, other issues associated with the method of this invention. A major issue is that of calibration; that is, the correction of amplitudes and phases among the transmitting smallsats and receiving smallsats, as well as the pointing of their antennas. An absence of adequate calibration could result in excessive angular sidelobe levels. However, similar calibration procedures are necessary in conventional phased-array antennas, so that the execution of such calibrations is not unusual. For the present method, a convenient calibration procedure can employ controlled RF transmissions from the aforementioned ground stations 104.

FIG. 4A, FIG. 4B, and FIG. 4C contain illustrative isometric drawings showing three alternative arrangements for adjacent smallsats in the present method. The arrangements depicted are termed, respectively, “docked,” “connected,” and “free-flying.” A docked arrangement, where adjacent smallsats are directly attached, is practical when the constellation design dictates small MIMO component spacings. A connected arrangement allows adjacent smallsats to be appropriately located using lightweight rigid tie rods 402; these tie rods could can be fixed or employ a “telescoping” feature to allow spacing changes for successive measurements or to facilitate storage prior to constellation deployment. For larger spacings, the free-flying arrangement may be more practical.

Each smallsat is ordinarily deployed in the form of a container 401, with a solar collector 403 and radar antenna 404 deployed after separation from the launch vehicle or space station, as the case may be.

Some bending, twisting, or warping of a constellation is to be anticipated and this could, if pronounced, adversely affect the radar beam patterns. To deal with this issue, a sensing capability is required to monitor the relative position and orientation of each smallsat. Such a capability may include, among other navigation approaches, appropriately-shaped laser beams 405 between adjacent smallsats impinging upon a pattern of photodetectors 406. Other sensing means exist: For example, U.S. Pat. No. 6,725,012 to Janson, et al, describes station-keeping approaches using differential GPS as well as by collecting RF transmissions from a cluster of subsatellites. A direct approach would use coherent RF or laser transmissions from ground stations, including the aforementioned ground station 104.

Orientation errors can be compensated by means of electronic or mechanical antenna pointing adjustments, depending on whether an electronic subarray or a swiveling mechanical pointing method is employed. Position errors, if minor, can be compensated by a phase-angle adjustment of the RF carrier on the affected smallsat(s), provided the phase-angle adjustment is a small fraction of the RF carrier period. When such adjustments are insufficient, small microthrusters 503, included on the smallsats, can be used to effect the needed corrections. Note that, in any case, such microthrusters or larger thrusters are essential for a free-flying arrangement to maintain the smallsat positions and orientation within an entire constellation.

FIG. 4A, FIG. 4B, and FIG. 4C each also includes a communications antenna 407 for a link connection to the central processor 204 and a GPS receiving antenna 408.

The drawings of FIG. 4A, FIG. 4B, and FIG. 4C should not be taken as excluding other smallsat arrangements in the present method.

FIG. 5A and FIG. 5B represent isometric views of the down-looking surfaces of the transmitting and receiving smallsats, respectively. Shown are components previously described, including containers 501, solar collectors 502, and microthrusters 503. The main purpose here is to describe the illustrative radar antennas 504, 505, 506. Shown, as an example, is a helical transmitting antenna 504 (Sakovsky, M., et al, “Rapid design of deployable antennas for cubesats,” IEEE Antennas and Propagation Magazine, pp. 50-58, April 2017) which transmits circular polarization signals, thereby avoiding the effects of Faraday rotation when transiting the earth's ionosphere. Often, dual-polarized observations are desired for space-based radar measurements, especially those for remote sensing of the environment. (Note that circular polarization is equivalent to two perpendicular linear polarizations, appropriately phased in quadrature.) For the present method, and using the aforementioned helical transmitting antenna 504, dual-polarized observations can be readily made using receiving antenna approaches such as the quadrifilar-helix design (Gao, S., et al, “Antennas for Modern Small Satellites,” IEEE Antennas and Propagation Magazine, pp. 40-56, August 2009), or more simply by employing dual endfire antennas oriented perpendicularly, as shown 505, 506. Also, these antennas 504, 505, 506 can be mechanically pointed in a desired direction and toward a selected surveillance sector with the aid of rotary joints 507.

It is emphasized that the antenna types depicted are purely illustrative and do not exclude many other pertinent types, such as deployable paraboloids (Sakovsky, M., et al, “Rapid design of deployable antennas for cubesats,” IEEE Antennas and Propagation Magazine, pp. 50-58, April 2017), membrane antennas (Cooley, M., “RF Design and Development of a Deployable Membrane Reflectarray Antenna for Space,” 2019 IEEE Phased Array Systems and Technology Symposium, Waltham, Mass., October 2019), or small phased arrays, from being employed for the present method.

The directive gain of a chosen antenna type plays an important role for both the detection sensitivity of the measurement and for the smallsat configuration parameters (such as smallsat spacings). The higher the directive gain, the further apart can be adjacent smallsats without incurring undesirable grating lobes (Cheston, T. C. and Frank, J., “Phased Array Radar Antennas,” Chapter 7 in Radar Handbook, 2nd Ed., M. I. Skolnik, Editor, McGraw-Hill, 1990)—and hence the finer the angular resolution. (The formulae of Cheston and Frank, cited above, related to grating lobe avoidance for two-way paths, are adjusted in the illustrative embodiment herein to apply to a one-way receiving path.)

As a practical matter, the individual transmitted RF average power levels are constrained by the smallsat weight and volume capacities. However, the aggregate total power is determined by the total number of transmitting smallsats, so that, for large constellations, it can be quite substantial and thereby advantageous for increasing the detection sensitivity.

The bounds of operation for the present method, for earth observations, include the following: (a) constellation altitudes from low earth orbit to geostationary altitude; (b) any orbit inclination; and (c) radar frequencies extending from high frequency (HF) through the laser radar (“lidar”) spectrum. (HF is normally not useful for surveillance of the earth from space; however it could be employed for topside “soundings” of regions of the earth's ionosphere or to probe beneath the surface of other planets.)

A closing comment is offered here: Those skilled in the art will be aware that an alternative method of achieving a fine angular resolution with a limited amount of actual radar RF hardware consists of the use of a Mills Cross (Slattery, B. R., “Use of Mills cross receiving arrays in radar systems,” Proc. IEE, pp. 1712-1722, November 1966), more widely employed in radio astronomy. In this latter method, the basic resolution is defined by the intersection of two fan beams. However, for measurements of regions on or above a planet's surface in a downlooking mode, it is extremely likely that a large number of echo sources (scatterers) will reside within specific range resolution “cells.” In these cases, for Q “real” echo sources there will result Q times (Q−1) false intersections (“ghosts”). For example, for ten detectable scatterers within a specific range cell over a surveillance sector, there will be ten real targets and 90 ghost targets. Techniques exist for eliminating these ghost responses, as discussed in Slattery's paper cited above, but they involve a great deal of additional complexity; in any case, those are not MIMO techniques.

Illustrative Embodiment of the Method

An illustrative embodiment of the present method will serve to broadly outline a smallsat-MIMO constellation design. The example is that of a scatterometer, of which many other types have been flown in the past to infer at-sea wind speeds by estimating sea state (i.e., wave height) on earth. The “baseline” configuration in FIG. 2A, employing orthogonal waveforms, simultaneous overlapping surveillance sectors, and a radar RF wavelength of 0.25 meter (in the L-band microwave region), are assumed.

The assumed operation employs a constellation of 50 transmitting and 100 receiving smallsats in a roughly downlooking mode from an orbit altitude of 500 km. The Doppler spread due to the constellation motion limits, in practical terms, individual pulse widths to about 20 microseconds, resulting in a waveform duty factor of approximately 1.5 percent for the low pulse repetition frequency (PRF) needed to avoid range ambiguities.

For this example, a single helical antenna is selected for each transmitting and receiving smallsat, with the transmitting and receiving helices spiraled in opposite directions in order to select principal, rather than opposite, received circular polarization. This provides an individual directive gain of 18.75 dBi (Sakovsky, M., et al, “Rapid design of deployable antennas for cubesats,” IEEE Antennas and Propagation Magazine, pp. 50-58, April 2017). This value of gain allows spacings of not more than 0.56 meter between virtual antennas (1.18-meter spacings for the actual smallsats), as limited by the need to avoid significant grating lobes. The result is a coherent virtual planar aperture with dimensions of approximately 50 by 25 meters, which is much larger than those of typical traditional individual spacecraft.

The result is a surveillance sector of a half-power diameter of about 230-km on the ocean's surface. The minimum grazing angle in this example is 60 degrees. The corresponding angular resolution “cell” size on the surface is roughly 5.0 km by 2.5 km (half-power) directly beneath the constellation, and somewhat degraded off the nadir. This offers a huge improvement of resolution over previous and current scatterometers (Raney, R. K., “Space-Based Remote Sensing Radars,” Chapter 16 in Radar Handbook, 3rd Ed., M. I. Skolnik, Editor, pp. 18.56-18.58, McGraw-Hill, 2009 and Florida State University, https://www.coaps.fsu.edu/scatterometry/about/overview/php), and is fine enough to observe localized squalls and to provide excellent estimations of wind fetch. Moreover, the persistent observations of many cells over a wide ocean area can allow research on the effects of distant swells and also allow a simultaneous comparison of echoes between upwind-downwind-crosswind conditions.

For a peak power, per pulse, of 36 watts for each transmitting smallsat (0.54-watt average power) and a coherent integration interval of 0.23 second (as limited by the decorrelation time of calm waves), it should be possible to detect, at the far end of the sector, a backscatter coefficient)(σ°) as small as −33 dB, which corresponds to calm sea states at 60-degree grazing angle (Nathanson, F. E., Chapter 7 in “Radar Design Principles,” 2nd Ed., McGraw-Hill, 1990). Higher sea states result in shorter decorrelation times, but also result in larger backscatter coefficients, thereby acing together to preserve the detection sensitivity for the chosen power levels, as contained in Nathanson's text, cited above.

Using the “rule of thumb” offered by Peral, E., et al, in “Radar technologies for earth remote sensing from cubesat platforms,” Proc. IEEE, pp. 404-418, March 2018, namely assignment of 5.5 kg mass per watt of average transmitted power by a smallsat, the aggregate mass of the constellation in this illustrative embodiment is estimated very conservatively as approximately several hundred kilograms. This represents a small fraction of what can be carried by a conventional large launch vehicle and also qualifies as a payload compatible with the use of a single small launch vehicle, of the type currently under development (Niederstrasser, C., “Small launch vehicles—a 2018 state of the industry survey,” 32nd Annual AIAA/USU Conference on Small Satellites, 2018).

The latter illustrative embodiment was presented at a 2019 phased-array symposium (Weissman, I., “Smallsat Surveillance Constellations Using MIMO Radar,” 2019 IEEE Phased Array Systems and Technology Symposium, Waltham, Mass., October 2019).

By extending this illustrative embodiment using the previous description, it should be understood that still finer resolution is obtainable. This can readily be achieved using smallsat antennas with even higher directive gain, such as by using, for example, unfurled paraboloid antennas. The higher gain permits wider smallsat spacings (conveniently, permitting larger antennas), resulting in finer resolution—but at the expense of a smaller surveillance sector.

REFERENCES CITED US Patent Documents

4,843,397 June 1989 Galati, et al 6,725,012 April 2004 Janson, et al 7,994,965 August 2011 Longstaff 8,427,360 April 2013 Longstaff 10,145,936 December 2018 Kishigami, et al 20110221625 September 2011 Comic, et al 20160204840 July 2016 Liu

Other Publications

-   Rahmat-Samii, Y., et al, “For Satellites, Think Small, Dream Big,”     IEEE Antennas and Propagation Magazine, pp. 22-30, April 2017 -   Bergin, J., and Guerci, J. R., “MIMO Radar: Theory and Application,”     Artech House, 2018 -   Davis, M. S. and Lanterman, A. D., “Coherent MIMO Radar: The Phased     Array and Orthogonal Waveforms,” IEEE Aerospace and Electronic     Systems Magazine, Part II of II, August 2014 -   Kilpatrick, T. and Longstaff, I. D., “Generalising the co-array, for     SAR and MIMO radar,” IEEE International Radar Conference Record,”     Washington, D.C., May 2015 -   Fuhrmann, D. R. and San Antonio, G., “Transmit Beamforming for MIMO     Radar Systems Using Partial Signal Correlation,” IEEE Thirty Eighth     Asilomar Conference on Signals, Systems and Computers, 2004 -   Steyskal, H., et al, “Pattern Synthesis for TechSat21—A Distributed     Space-Based Radar System,” IEEE Antennas and Propagation Magazine,     pp. 19-25, August 2003 -   Sakovsky, M., et al, “Rapid design of deployable antennas for     cubesats,” IEEE Antennas and Propagation Magazine, pp. 50-58, April     2017 -   Cheston, T. C. and Frank, J., “Phased Array Radar Antennas,” Chapter     7 in Radar Handbook, 2nd Ed., M. I. Skolnik (Editor), McGraw-Hill,     1990 -   Gao, S., et al, “Antennas for Modern Small Satellites,” IEEE     Antennas and Propagation Magazine, pp. 40-56, August 2009 -   Cooley, M., “RF Design and Development of a Deployable Membrane     Reflectarray Antenna for Space,” 2019 IEEE Phased Array Systems and     Technology Symposium, Waltham, Mass., October 2019 -   Slattery, B. R., “Use of Mills cross receiving arrays in radar     systems,” Proc. IEE, pp. 1712-1722, November 1966 -   Raney, R. K., “Space-Based Remote Sensing Radars,” Chapter 16 in     Radar Handbook, 3rd Ed., M. I. Skolnik (Editor), pp. 18.56-18.58,     McGraw-Hill, 2009 Florida State University,     https://www.coaps.fsu.edu/scatterometry/about/overview/php -   Nathanson, F. E., Chapter 7 in “Radar Design Principles,” 2nd Ed.,     McGraw-Hill, 1990 -   Peral, E., et al, “Radar technologies for earth remote sensing from     cubesat platforms,” Proc. IEEE, pp. 404-418, March 2018 -   Niederstrasser, C., “Small launch vehicles—a 2018 state of the     industry survey,” 32nd Annual AIAA/USU Conference on Small     Satellites, 2018 -   Weissman, I., “Smallsat Surveillance Constellations Using MIMO     Radar,” 2019 IEEE Phased Array Systems and Technology Symposium,     Waltham, Mass., October 2019 

I claim:
 1. A method for providing high-resolution electromagnetic measurements of targets within a spatial region of above, below, and upon the surface of a planet, comprising: installing a plurality of smallsats in orbit above said spatial region, wherein each of said smallsats contains at least one of a transmitter and receiver, said transmitter capable of transmitting electromagnetic signals to said spatial region, and said receiver capable of receiving electromagnetic signals reflected from said spatial region; implementing a multiple input, multiple output arrangement, wherein a plurality of receiving smallsats, each capable of receiving a plurality of said electromagnetic signals reflected from said spatial region, thereby creating a virtual aperture of receiving elements; generating a plurality of said transmitted electromagnetic signals that are substantially orthogonal; post-processing, using matched filters, a plurality of said received electromagnetic signals that are substantially orthogonal; whereby at least one high-resolution electromagnetic measurement may be obtained of targets within said spatial region.
 2. The method of claim 1 wherein said plurality of smallsats comprises a plurality of radio frequency transmitting smallsats arranged in at least one array, and said plurality of radio frequency receiving smallsats arranged in at least one array.
 3. The method of claim 2 wherein said arrays are physically separated in their entirety.
 4. The method of claim 1 wherein the spacings between adjacent smallsats are kept small enough to avoid significant grating lobes in the beam patterns, thereby maintaining said virtual aperture in a substantially unthinned configuration.
 5. The method of claim 1 wherein the relative positions and orientations of said smallsats within said constellation remain substantially fixed for the duration of a measurement.
 6. The method of claim 1 wherein means are employed to sense and correct for unintended perturbations of smallsat positions and orientations.
 7. The method of claim 1 wherein said smallsats employ means for varying the beam pointing directions.
 8. The method of claim 1 wherein said smallsats provide means for dual polarization operation.
 9. The method of claim 1 wherein at least one of said steps of generating and post-processing is carried out in a processor satellite that accompanies said plurality of smallsats in orbit.
 10. The method of claim 9 wherein at least one base station is established for communicating with said processor satellite and for receiving measurements therefrom.
 11. A system for providing high-resolution electromagnetic measurements of targets within a spatial region of above, below, and upon the surface of a planet, comprising: installing a plurality of smallsats in orbit above said spatial region, wherein each of said smallsats contains at least one of a transmitter and receiver, said transmitter capable of transmitting electromagnetic signals to said spatial region, and said receiver capable of receiving electromagnetic signals reflected from said spatial region; implementing a multiple input, multiple output arrangement, wherein a plurality of receiving smallsats, each capable of receiving a plurality of said electromagnetic signals reflected from said spatial region, thereby creating a virtual aperture of receiving elements; generating a plurality of said transmitted electromagnetic signals that are substantially orthogonal; post-processing, using matched filters, a plurality of said received electromagnetic signals that are substantially orthogonal; whereby at least one high-resolution electromagnetic measurement may be obtained of targets within said spatial region.
 12. The system of claim 11 wherein said plurality of smallsats comprises a plurality of radio frequency transmitting smallsats arranged in at least one array, and said plurality of radio frequency receiving smallsats arranged in at least one array.
 13. The system of claim 12 wherein said arrays are physically separated in their entirety.
 14. The system of claim 11 wherein the spacings between adjacent smallsats are kept small enough to avoid significant grating lobes in the beam patterns, thereby maintaining said virtual aperture in a substantially unthinned configuration.
 15. The system of claim 11 wherein the relative positions and orientations of said smallsats within said constellation remain substantially fixed for the duration of a measurement.
 16. The system of claim 11 wherein means are employed to sense and correct for unintended perturbations of smallsat positions and orientations.
 17. The system of claim 11 wherein said smallsats employ means for varying the beam pointing directions.
 18. The system of claim 11 wherein said smallsats provide means for dual polarization operation.
 19. The system of claim 11 wherein at least one of said steps of generating and post-processing is carried out in a processor satellite that accompanies said plurality of smallsats in orbit.
 20. The system of claim 19 wherein at least one base station is established for communicating with said processor satellite and for receiving measurements therefrom. 